The delivery of drug species to the brain is oft-times seriously limited by transport and metabolism factors and, more specifically, by the functional barrier of the endothelial brain capillary wall, i.e. the blood-brain barrier or BBB. Site-specific delivery and sustained delivery of drugs to the brain are even more difficult.
Indeed, the barriers separating plasma from the brain and cerebrospinal fluid (CSF) are complex systems involving passive and active transport and serve a number of important functions. The boundary between plasma and the central nervous system (CNS) is much less permeable than that between plasma and other tissue cells to a variety of water soluble substances, such as organic electrolytes, organic acids and bases, as well as to large molecules such as proteins. Such a barrier also provides a path for clearance from the brain of the breakdown products of cellular metabolism. The CNS and its fluids can be considered basically a three-compartment system: the blood or the plasma, CSF and brain tissue. There is a diffusion-controlled exchange between CSF and the extracellular fluid (CF) of the brain. It has also been suggested that the permeabilities of blood-CSF and blood-brain barriers are practically identical with respect to drugs and other foreign substances. Mayer et al, J. Pharmacol. and Exp. Therap., 125, 185 (1959).
The BBB is, moreover, basically the result of the fact that the endothelial cells in the brain capillaries are joined by continuous, tight intercellular junctions, such that material has to pass through the cells rather than between them in order to move from blood to brain. It is interesting that there are areas within the brain, such as the subfornical body and the postremia, in which the capillary cells are not closely linked so that they lack the characteristics of the BBB. They provide the entry of small amounts of compounds which would not ordinarily enter the barriers. Hoffman and Olszewzki, Neurology (Minneap.), 11, 1081 (1961).
Foreign compounds which enter organs other than the central nervous system with ease, may penetrate the CNS slowly or hardly at all. A number of theories concerning the nature of the barrier have been proposed. The widely accepted concept describes the boundary as a fat-like layer interspersed with small pores, although the BBB is not a simple, anatomically well-defined unitary physical entity. Shuttleworth, Prog. Exp. Tumor Res., 17, 279 (1972). Penetration of such a barrier may occur by several processes: lipid soluble substances may passively penetrate into the cells, while small molecules such as water and urea may pass through the pores. In addition to these simple physical processes, carrier-mediated and active transport processes govern the movement of many molecules through the BBB. Thus, it is generally accepted that lipid solubility, degree of ionic dissociation or protonation and the ability of temporary combination with membrane constituents affect delivery through the BBB. It has been shown, for example, that in the class of barbiturates, a quantitative correlation could be established between their ease to pass into the brain (as reflected by the different times of onset of anesthetic action) and their lipid/water partition coefficient. Mark et al, J. Pharmacol. and Exp. Therap., 123, 79 (1957). The role of lipid solubility in drug penetration through the BBB is also exemplified by the better absorption of the sparingly water-soluble thiamine propyl disulfide (TPD) as compared to the water-soluble thiamine hydrochloride (THCl). Thomson et al, Ann, Int. Med., 74, 529 (1971). Some materials such as glucose and amino acids are transported by active mechanism, characterized by saturation, bidirectional molecular specificity, bidirectional competitive inhibition and bidirectional countertransport. Fishman, Am. J. Physiol., 206, 836 (1964).
Changes in permeability of the BBB can be caused by several pathological and toxicological processes. Pardridge, Connor and Crawford, CRC Crit. Rev. Toxicol., 179 (1975). A general increase in the barrier permeability, such as a nonspecific breakdown of the barrier has, however, several consequences, including cerebral edema.
It too is well documented that the BBB is relatively impermeable to the ionized forms of drugs and other molecules. Drugs which are weak organic electrolytes appear to pass from blood to CSF to reach a steady state ratio characteristic of each molecule according to its pK.sub.a and the existence of a normal pH gradient between blood and CSF. It is clear that it is the most difficult for quaternary pyridinium or ammonium salts to penetrate the BBB.
And removal of substances from the brain and CSF is obviously a significant factor in regulating drug concentrations in the CNS. There are several efflux processes: bulk flow via the arachnoid villi, diffusion of lipid soluble substances into brain and blood, active transport and metabolism by adjacent meninges. Once a drug or metabolite enters the CSF from blood or brain by simple diffusion, it may rapidly be removed, either by nonselective bulk flow or by active transport mechanism associated with the choroid plexus or other nondefined structures in the CSF compartment. It is generally accepted that highly lipid-soluble drugs leave the CSF more rapidly than poorly lipid-soluble ones, but the barrier to passage of compounds from CSF has only superficial similarity to the blood-CSF barrier.
Drug elimination processes from the brain are significantly directly related to drug accumulation in the brain. It is generally assumed that efflux in the opposite direction involves almost the same processes as for entry, except that the role of the bulk flow and the metabolic processes in the brain are not to be overlooked.
The two elimination processes studied in the earlier literature and which can be said to have a certain bearing on the present invention involve elimination from the brain of ionic species. Thus, it is found that non-metabolized ionic species, such as the acetate ion, have a three times slower elimination rate from the CSF than from the blood. Freundt, Arz. Forsch., 23, 949 (1973). An even more dramatic change in the elimination rate was found in the case of a quaternary piperidinium salt. The quaternary salt, formed in situ after delivery of a haloalkylamine, which undergoes cyclization to the quaternary salt in the brain as well, was found to have an at least ten times slower elimination rate from the brain than from the rest of the body. It was concluded by the authors [Ross and Froden, Eur. J. Pharmacol., 13, 46 (1970) ] that the outflow rate of the quaternary salt corresponded to the inflow rate. Similar results were obtained for the erythrocytes: the efflux of the quaternary salt was very slow. Ross, J. Pharm. Pharmacol., 27, 322 (1975).
A dihydropyridine.revreaction.pyridinium redox system has recently been successfully applied to delivery to the brain of a number of drugs. Generally speaking, according to this system, a dihydropyridine derivative of a biologically active compound is synthesized, which derivative can enter the CNS through the blood-brain barrier following its systemic administration. Subsequent oxidation of the dihydropyridine species to the corresponding pyridinium salt leads to delivery of the drug to the brain.
Three main approaches have been used thus far for delivering drugs to the brain using this redox system. The first approach involves derivation of selected drugs which contain a pyridinium nucleus as an integral structural component. This approach was first applied to delivering to the brain N-methylpyridinium-2-carbaldoxime chloride (2-PAM), the active nucleus of which constitutes a quaternary pyridinium salt, by way of the dihydropyridine latentiated prodrug form thereof. Thus, a hydrophilic compound (2-PAM) was made lipoidal (i.e. lipophilic) by making its dihydropyridine form (Pro-2-PAM) to enable its penetration through lipoidal barriers. This simple prodrug approach allowed the compound to get into the brain as Well as other organs, but this manipulation did not and could not result in any brain specificity. On the contrary, such approach was delimited to relatively small molecule quaternary pyridinium ring-containing drug species and did not provide the overall ideal result of brain-specific, sustained release of the desired drug, with concomitant rapid elimination from the general circulation, enhanced drug efficacy and decreased toxicity. No "trapping" in the brain of the 2-PAM formed in situ resulted, and obviously no brain-specific, sustained delivery occurred as any consequence thereof: the 2-PAM was eliminated as fast from the brain as it was from the general circulation and other organs. Compare U.S. Pat. Nos. 3,929,813 and 3,962,447; Bodor et al, J. Pharm. Sci., 67, No. 5, 685 (1978). See also Bodor, "Novel Approaches for the Design of Membrane Transport Properties of Drugs", in Design of Biopharmaceutical Properties Through Prodrugs and Analogs, Roche, E. B. (ed.), APhA Academy of Pharmaceutical Sciences, Washington, D.C., 98-135 (1976). Subsequent extension of this first approach to delivering a much larger quaternary salt, berberine, to the brain via its dihydropyridine prodrug form was, however, found to provide site-specific sustained delivery to the brain of that anti-cancer agent. See Bodor et al, Science, Vol. 214, Dec. 18, 1981, pp. 1370-1372.
The second approach for delivering drugs to the brain using the redox system involves the use of a dihydropyridine/pyridinium carrier chemically linked to a biologically active compound. Bodor et al, Science, Vol. 214, Dec. 18, 1981, pp. 1370-1372, outlines a scheme for this specific and sustained delivery of drug species to the brain, as depicted in the following Scheme 1: ##STR1## According to the scheme in Science, a drug [D] is coupled to a quaternary carrier [QC].sup.+ and the [D-QC].sup.+ which results is then reduced chemically to the lipoidal dihydro form [D-DHC]. After administration of [D-DHC] in vivo, it is rapidly distributed throughout the body, including the brain. The dihydro form [D-DHC] is then in situ oxidized (rate constant, k.sub.1) (by the NAD.revreaction.NADH system) to the ideally inactive original [D-QC].sup.+ quaternary salt which, because of its ionic, hydrophilic character, should be rapidly eliminated from the general circulation of the body, while the blood-brain barrier should prevent its elimination from the brain (k.sub.3 &gt;&gt;k.sub.2 ; k.sub.3 &gt;&gt;k.sub.7). Enzymatic cleavage of the [D-QC].sup.+ that is "locked" in the brain effects a sustained delivery of the drug species [D], followed by its normal elimination (k.sub.5), metabolism. A properly selected carrier [QC].sup.+ will also be rapidly eliminated from the brain (k.sub.6 &gt;&gt;k.sub.2). Because of the facile elimination of [D-QC].sup.+ from the general circulation, only minor amounts of drug are released in the body (k.sub.3 &gt;&gt;k.sub.4); [D] will be released primarily in the brain (k.sub.4 &gt;k.sub.2). The overall result ideally will be a brain-specific sustained release of the target drug species. Specifically, Bodor et al worked with phenylethylamine as the drug model. That compound was coupled to nicotinic acid, then quaternized to give compounds of the formula ##STR2## which were subsequently reduced by sodium dithionite to the corresponding compounds of the formula ##STR3## Testing of the N-methyl derivative in vivo supported the criteria set forth in Scheme 1. Bodor et al speculated that various types of drugs might possibly be delivered using the depicted or analogous carrier systems and indicated that use of N-methylnicotinic acid esters and amides and their pyridine ring-substituted derivatives was being studied for delivery of amino- or hydroxyl-containing drugs, including small peptides, to the brain. No other possible specific carriers were disclosed. Other reports of this work with the redox carrier system have appeared in The Friday Evening Post, Aug. 14, 1981, Health Center Communications, University of Florida, Gainesville, Fla.; Chemical & Engineering News, Dec. 21, 1981, pp. 24-25; and Science News, Jan. 2, 1982, Vol. 121, No. 1, page 7. More recently, the redox carrier system has been substantially extended in terms of possible carriers and drugs to be delivered. See International Patent Application No. PCT/US83/00725, filed May 12, 1983 and published Nov. 24, 1983 under International Publication No. WO83/03968. Also see Bodor et al, Pharmacology and Therapeutics, Vol. 19, No. 3, pp. 337-386 (1983); and Bodor U.S. Pat. No. 4,540,564, issued Sep. 10, 1985.
The aforementioned Bodor U.S. Pat. No. 4,540,564 specifically contemplates application of the dihydropyridine.revreaction.pyridinium salt carrier system to amino acids and peptides, particularly small peptides having 2 to 20 amino acid units. Among the amino acids and peptides mentioned in the patent are GABA, tyrosine, tryptophan, met.sup.5 -enkephalin, leu.sup.5 -enkephalin, LHRH and its analogs and others. Representative carrier-linked amino acids and peptides illustrated in the Bodor patent are the following: ##STR4## Thus, in the depicted carrier system as applied to amino acids and peptides, the free carboxyl function is suitably protected to prevent premature metabolism while the trigonelline-type carrier is linked to the amino acid or peptide through its free amino function. Oxidation of the dihydropyridine carrier moiety in vivo to the ionic pyridinium salt carrier/drug entity prevents elimination thereof from the brain, while elimination from the general circulation is accelerated, and subsequent cleavage of the quaternary carrier/drug species results in sustained delivery of the amino acid or peptide (e.g. tryptophan, GABA, leu.sup.5 -enkephalin, etc.) in the brain and facile elimination of the carrier moiety.
The third approach for delivering drugs to the brain using the redox system provides derivatives of centrally acting amines in which a primary, secondary or tertiary amine function has been replaced with a dihydropyridine/pyridinium salt redox system. These brain-specific analogs of centrally acting amines have been recently described in International Patent Application No. PCT/US85/00236, filed Feb. 15, 1985 and published Sep. 12, 1985 under International Publication No. WO85/03937. The dihydropyridine analogs are characterized by the structural formula ##STR5## wherein D is the residue of a centrally acting primary, secondary or tertiary amine, and ##STR6## is a radical of the formula ##STR7## wherein the dotted line in formula (a) indicates the presence of a double bond in either the 4 or 5 position of the dihydropyridine ring; the dotted line in formula (b) indicates the presence of a double bond in either the 2 or 3 position of the dihydroquinoline ring system; m is zero or one; n is zero, one or two; p is zero, one or two, provided that when p is one or two, each R in formula (b) can be located on either of the two fused rings; q is zero, one, or two, provided that when q is one or two, each R in formula (c) can be located on either of the two fused rings; and each R is independently selected from the group consisting of halo, C.sub.1 -C.sub.7 alkyl, C.sub.1 -C.sub.7 alkoxy, C.sub.2 -C.sub.8 alkoxycarbonyl, C.sub.2 -C.sub.8 alkanoyloxy, C.sub.1 -C.sub.7 haloalkyl, C1-C.sub.7 alkylthio, C.sub.1 -C.sub.7 alkylsulfinyl, C1-C.sub.7 alkylsulfonyl, --CH.dbd.NOR'" wherein R'"is H or C.sub.1 -C.sub.7 alkyl, and --CONR'R" wherein R' and R", which can be the same or different, are each H or C.sub.1 -C.sub.7 alkyl. These dihydropyridine analogs act as a delivery system for the corresponding biologically active quaternary compounds in vive. Due to its lipophilic nature, the dihydropyridine analog will distribute throughout the body and has easy access to the brain through the blood-brain barrier. Oxidation in vivo will then provide the quaternary form, which will be "locked" preferentially in the brain. In contradistinction to the drug-carrier entities described in Bodor U.S. Pat. No. 4,540,564 and related publications, however, there is no readily metabolically cleavable bond between drug and quaternary portions, and the active species delivered is not the original drug from which the dihydro analog was derived, but rather is the quaternary analog itself.
The aforementioned International Publication No. WO85/03937 contemplates application of its analog system to amino acids and small peptides, e.g. the enkephalins, tryptophan, GABA, LHRH analogs and others. Illustrated redox analogs include the following: ##STR8## In the depicted analog system as applied to amino acids and peptides, the free carboxyl function is thus suitably protected to prevent premature metabolism while the dihydropyridine.revreaction.pyridinium salt type redox system replaces the free amino function in the amino acid or peptide.
As described in International Publication No. WO85/03937, the chemical processes for preparing the redox analog derivatives replace any free amino function in the selected drug with the redox analog system. When these processes are applied to amino acids, they provide a redox amino acid which no longer contains a free amino function for linkage to another amino acid or peptide via a peptide bond (--CONH--). Such an analog amino acid can thus only be used to prepare a peptide having the analog amino acid located at the peptide's N-terminus. This-severely limits use of the redox analog amino acids in peptide synthesis. It therefore would be desirable to provide a new approach for delivering peptides to the brain using the redox system, which approach would provide novel redox amino acids which could be used to synthesize peptides having the redox system inserted at a variety of locations in the peptide chain.